Ink jet printing involves ejecting ink droplets from orifices in a print head onto a receiving substrate to form an image. Ink-jet printing systems commonly utilize either direct printing or offset printing architecture. In a typical direct printing system, ink is ejected from jets in the print head directly onto the final receiving substrate. In an offset printing system, the print head jets the ink onto an intermediate transfer surface, such as a liquid layer on a drum. The final receiving substrate is then brought into contact with the intermediate transfer surface and the ink image is transferred and fused or fixed to the substrate.
Alignment of the print head within an ink jet printing system including a single print head may be expressed as the position of the print head relative to the image receiving surface. Alignment of multiple print heads in ink jet printing systems including multiple print heads may be expressed as the position of one print head relative to the image receiving surface, such as a media substrate or intermediate transfer surface, or another print head within a coordinate system of multiple axes. For purposes of discussion, the terms “cross-process direction” and “X-axis direction” refer to a direction or axis perpendicular to the direction of travel of an image receiving surface past a print head, the terms “process direction” and “Y-axis direction” refer to a direction or axis parallel to the direction of an the image receiving surface, the term “Z-axis” refers to an axis perpendicular to the X-Y axis plane.
One particular type of alignment parameter is print head roll. As used herein, print head roll refers to clockwise or counterclockwise rotation of a print head about an axis normal to the image receiving surface, i.e., Z-axis. Print head roll misalignment may result from factors such as mechanical vibrations, and other sources of disturbances on the machine components, that may alter print head positions and/or angles with respect to an image receiving surface. As a result of roll misalignment, the rows of nozzles may be arranged diagonally with respect to the process direction movement of the image receiving surface as a result of the roll of the print head, which may cause horizontal lines, image edges, and the like to be skewed relative to the image receiving surface.
One method that may be used to detect print head roll is printing a horizontal line using one or more rows of nozzles of a print head and measuring the angle of the one or more lines with respect to the horizontal using a flatbed scanner or inline linear array sensor. The angle measurements may then be used to detect print head roll. Measuring angles of printed lines, however, requires precise alignment of the scanner or sensor with respect to the image receiving surface. If the measurement system uses a printed sheet on a flatbed scanner, rotation of the paper with respect to the scanner may produce inaccurate measurements. Similarly, if the measurement system utilizes an inline linear array sensor, misalignment of the sensor with respect to the image receiving surface may produce inaccurate measurements.